[tip: x86/core] objtool: Add option to generate prefix symbols

["tip-bot2 for Peter Zijlstra" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     9f2899fe36a623885d8576604cb582328ad32b3c
Gitweb:        https://git.kernel.org/tip/9f2899fe36a623885d8576604cb582328ad32b3c
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Fri, 28 Oct 2022 15:50:42 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Nov 2022 13:44:09 +01:00

objtool: Add option to generate prefix symbols

When code is compiled with:

  -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES}

functions will have PADDING_BYTES of NOP in front of them. Unwinders
and other things that symbolize code locations will typically
attribute these bytes to the preceding function.

Given that these bytes nominally belong to the following symbol this
mis-attribution is confusing.

Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to
claim these bytes, allow objtool to emit __pfx_##name symbols to do
the same.

Therefore add the objtool --prefix=N argument, to conditionally place
a __pfx_##name symbol at N bytes ahead of symbol 'name' when: all
these preceding bytes are NOP and name-N is an instruction boundary.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.526899822@infradead.org
---
 tools/objtool/builtin-check.c           |  1 +-
 tools/objtool/check.c                   | 33 +++++++++++++++++++++++-
 tools/objtool/elf.c                     | 31 +++++++++++++++++++++++-
 tools/objtool/include/objtool/builtin.h |  1 +-
 tools/objtool/include/objtool/elf.h     |  2 +-
 5 files changed, 67 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 0a04f8e..95fcece 100644
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -75,6 +75,7 @@ const struct option check_options[] = {
 	OPT_BOOLEAN('r', "retpoline", &opts.retpoline, "validate and annotate retpoline usage"),
 	OPT_BOOLEAN(0,   "rethunk", &opts.rethunk, "validate and annotate rethunk usage"),
 	OPT_BOOLEAN(0,   "unret", &opts.unret, "validate entry unret placement"),
+	OPT_INTEGER(0,   "prefix", &opts.prefix, "generate prefix symbols"),
 	OPT_BOOLEAN('l', "sls", &opts.sls, "validate straight-line-speculation mitigations"),
 	OPT_BOOLEAN('s', "stackval", &opts.stackval, "validate frame pointer rules"),
 	OPT_BOOLEAN('t', "static-call", &opts.static_call, "annotate static calls"),
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 7936312..27f35f5 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3417,7 +3417,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
 			/* Ignore KCFI type preambles, which always fall through */
-			if (!strncmp(func->name, "__cfi_", 6))
+			if (!strncmp(func->name, "__cfi_", 6) ||
+			    !strncmp(func->name, "__pfx_", 6))
 				return 0;
 
 			WARN("%s() falls through to next function %s()",
@@ -3972,6 +3973,34 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio
 	return false;
 }
 
+static int add_prefix_symbol(struct objtool_file *file, struct symbol *func,
+			     struct instruction *insn)
+{
+	if (!opts.prefix)
+		return 0;
+
+	for (;;) {
+		struct instruction *prev = list_prev_entry(insn, list);
+		u64 offset;
+
+		if (&prev->list == &file->insn_list)
+			break;
+
+		if (prev->type != INSN_NOP)
+			break;
+
+		offset = func->offset - prev->offset;
+		if (offset >= opts.prefix) {
+			if (offset == opts.prefix)
+				elf_create_prefix_symbol(file->elf, func, opts.prefix);
+			break;
+		}
+		insn = prev;
+	}
+
+	return 0;
+}
+
 static int validate_symbol(struct objtool_file *file, struct section *sec,
 			   struct symbol *sym, struct insn_state *state)
 {
@@ -3990,6 +4019,8 @@ static int validate_symbol(struct objtool_file *file, struct section *sec,
 	if (!insn || insn->ignore || insn->visited)
 		return 0;
 
+	add_prefix_symbol(file, sym, insn);
+
 	state->uaccess = sym->uaccess_safe;
 
 	ret = validate_branch(file, insn_func(insn), insn, *state);
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 36dc787..3d636d1 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -819,6 +819,37 @@ elf_create_section_symbol(struct elf *elf, struct section *sec)
 	return sym;
 }
 
+static int elf_add_string(struct elf *elf, struct section *strtab, char *str);
+
+struct symbol *
+elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size)
+{
+	struct symbol *sym = calloc(1, sizeof(*sym));
+	size_t namelen = strlen(orig->name) + sizeof("__pfx_");
+	char *name = malloc(namelen);
+
+	if (!sym || !name) {
+		perror("malloc");
+		return NULL;
+	}
+
+	snprintf(name, namelen, "__pfx_%s", orig->name);
+
+	sym->name = name;
+	sym->sec = orig->sec;
+
+	sym->sym.st_name = elf_add_string(elf, NULL, name);
+	sym->sym.st_info = orig->sym.st_info;
+	sym->sym.st_value = orig->sym.st_value - size;
+	sym->sym.st_size = size;
+
+	sym = __elf_create_symbol(elf, sym);
+	if (sym)
+		elf_add_symbol(elf, sym);
+
+	return sym;
+}
+
 int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,
 			  unsigned long offset, unsigned int type,
 			  struct section *insn_sec, unsigned long insn_off)
diff --git a/tools/objtool/include/objtool/builtin.h b/tools/objtool/include/objtool/builtin.h
index 22092a9..f341b62 100644
--- a/tools/objtool/include/objtool/builtin.h
+++ b/tools/objtool/include/objtool/builtin.h
@@ -26,6 +26,7 @@ struct opts {
 	bool stackval;
 	bool static_call;
 	bool uaccess;
+	int prefix;
 
 	/* options: */
 	bool backtrace;
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 9e96a61..b6974e3 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -146,6 +146,8 @@ static inline bool has_multiple_files(struct elf *elf)
 struct elf *elf_open_read(const char *name, int flags);
 struct section *elf_create_section(struct elf *elf, const char *name, unsigned int sh_flags, size_t entsize, int nr);
 
+struct symbol *elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size);
+
 int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
 		  unsigned int type, struct symbol *sym, s64 addend);
 int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,